Optical sensors are commonly employed in a wide variety of applications including among others, inertial navigation sensors systems where an optical sensor is responsive to rotation thereof, and optical current and voltage sensors where the optical sensor is responsive to electric and/or magnetic fields. Each of these optical sensors employs an optical beam or light wave that propagates along a specific optical path. Each of these optical sensors generally includes light detector circuitry and signal processing circuitry for sensing a specific parameter that affects the optical beam along the optical path. More specifically, the propagating light wave is affected by the intended parameter intended to be monitored or sensed, for example rotation of the optical path for inertial sensors, electric fields for voltage sensors, and magnetic or electric fields for current sensors, and the like. The characteristic behavior of the light wave or optical beam, for example velocity, phase, and/or polarization may be affected by the intended parameter to be sensed in accordance with well-known and established physical principles.
Commonly, optical sensor systems of the types described employ a photo-detector for obtaining an indication of the instantaneous intensity of one or more optical beams, either separately or in combination, impinging on the photo-detector. In turn the photo-detector output signal is signal processed by a signal processing circuit for deriving therefrom an indication of the parameter intended to be measured—the parameter affecting the optical beam, e.g., the magnitude of the of the change in velocity, phase, and/or polarization in accordance with well known and established principles. Further, the signal processing circuit may also include one or more outputs that may be utilized as a drive signal for affecting the optical path and thereby be incorporated into a closed-loop feedback signal processing sensor scheme for obtaining the indication of the parameter intended to be measured as is in accordance with well known and established principles.
A relatively new application of optical sensors is high voltage power line monitoring systems. These systems require highly accurate current and voltage sensors in which the sensors are physically immersed in the brutal electrical environment of high voltage power lines where the line voltage is in the order of hundreds of Kilo-volts and power line current may be in the range of 1.0 to 5000.0 amps. The latter wide dynamic current range needed to be measured by current sensors is a severe problem in the power line application.
The wide dynamic range requirement may be understood by considering that power plants coupled to an electrical power line grid may be operative in one scenario providing peak current output during heavy summertime use due to air conditioning equipment, i.e., 5000 amps. In another scenario, the power plant may be in an idling mode where the power plant acts as a small load receiving a small amount of current, or is putting out a small amount of current, say, in the order of 1.0 amp.
This wide dynamic range accuracy requirement has commonly necessitated the use of multiple sets of instrumentation to measure power line currents during the aforesaid scenarios. For example, transformers employed for obtaining current measurements may include multiple taps where each tap may be coupled to two or more metering instrumentation systems or meters.
Optical current sensors, and more particularly fiber optic current sensors, are now employed in power-line applications for measuring wide dynamic range current flowing through the power lines. Principles of these types of optical sensors are taught in U.S. Pat. No. 5,644,397, entitled, “Fiber Optic Interferometric Circuit And Magnetic Field Sensor,” issued to James N. Blake. Disclosed therein are both Sagnac interferometric and in-line embodiments for constructing an optical current sensor. Various improvements thereof are also disclosed, among others, in U.S. Pat. Nos. 5,696,858, 5,987,195, 6,023,331, and 6,122,415, all issued to James N. Blake.
Briefly, fiber optic current sensors as disclosed in the aforementioned patents work on the principal that a magnetic field produced by a current to be sensed affects the polarization properties of a sensing fiber in the vicinity of the magnetic field through the Faraday effect. The change in the polarization properties of the sensing fiber can be probed in several different ways. Common ways include injecting linearly polarized light and later analyzing the rotation of its polarization state after exiting the sensing region, or measuring the relative velocities of right and left hand circularly polarized light waves that travel through the sensing region using a Sagnac or in-line interferometer technique. The sensor configuration includes an optical exit port for permitting an “affected” optical beam to exit the optical circuit and impinge upon a photo-detector positioned at the terminus of the optical circuit. In all cases, the current to be sensed causes light intensity fluctuations of the exiting optical beam, and in turn causes the photo-detector output signal to exhibit fluctuations related to the light intensity fluctuations thereby providing a signal which may be processed by a signal processing circuit to provide a signal indicative of the current intended to be sensed.
The fluctuations of the light intensity falling on the photo-detector may be processed by analog circuitry to produce an analog output representative of the current being sensed. However, it is often more desirable to digitally process the light intensity fluctuations. Digital processing is often more desirable because: (i) digital output is often more desirable for following subsystems; (ii) more accuracy may be obtained with digital signal processing because the higher processing power available in the digital domain allows for complex characterization, (iii) it is much cheaper to carry out complex signal processing in the digital domain rather than in the analog domain, and (iv) wide dynamic range signals may be more accurately handled in the digital domain.
Generally, the first step required for processing the photo-detector signal in the digital domain is to convert the signal using an A/D converter. After the signal has been processed, it may be converted back to the analog domain using a D/A converter. This conversion back to the analog domain is often required for current and voltage sensors as these sensors may be married to secondary or receiving devices (such as meters, relays, and recorders) which in the power industry often have analog system front-ends.
It should be noted that these secondary devices also have a wide-dynamic range requirement. For example, consider a power meter for measuring the product of sensed current and voltage where the current ranges, as before between 1.0 and 5000.0 amps. Commonly such power meters include analog-to-digital converters and digital signal processing for deriving the desired information, for example watt-hours. As indicated earlier, multiple sets of instrumentation, i.e., multiple meters, may be employed to obtain accurate information depending on the current.
The absolute accuracy of these types of optical sensors as well as the secondary devices, i.e., meters, may be compromised by the A/D and D/A converters employed therewith. This may happen because of the quantization and non-linearities present in the A/D and D/A converters. As an example, consider what happens in an optical current sensor that is made to measure currents as low as ±1 amp, and as high as ±5000 amps. Suppose a 12-bit A/D converter is used to convert the photodetector output signal to a digital signal. These 12 bits (4096 distinct levels) have to describe a 10,000 amp range, or on average, each bit represents about 2.5 amps. Thus the 1 amp signal falls within 1 least significant bit (LSB). Normally, some noise will exist in the detected photo-detector signal that serves to “dither” the detected signal around several LSB's. This noise may be used to overcome the quantization error associated with the signal being comparable to or less than an LSB. However, the overall system accuracy is often not good enough even when the signal with noise spans several LSB's. This is so since the bit spacing for these several LSB's may not be representative of the overall bit spacing for the A/D converter. If these bits are closer together than the overall average bit spacing, then the final output signal will read relatively higher than a large signal which uses many of the bits; and, if these bits are further apart than the average bit spacing, then the final output signal will read relatively lower than a large signal that uses many of the bits.
The same problem as just described for the A/D converter also applies to the D/A converter. Larger converters such as 16-bit or higher A/D or D/A may be used to reduce the magnitude of this problem, but to meet the demanding specifications of wide dynamic range optical sensors with present day (typically 16 bit or less) A/D and D/A converters, there is a need for a signal processing circuit that diminishes the effects of bit non-linearities in these types of optical sensors, including voltage and current sensors, requiring large dynamic range.